Wheelchair users who spend a large portion of their day seated in a wheelchair are at risk of developing pressure ulcers (i.e., skin tissue breakdown) in their buttocks region. Pressure ulcers tend to occur in skin tissue that contacts the seat cushion near the bony prominences of the hip. Regions most prone to developing pressure ulcers include skin surrounding the ischial tuberosities, coccyx, and sacral area. A disabled wheelchair user, particularly one with partial or complete paralysis, is more likely to develop pressure ulcers because he or she lacks supportive muscular tissue that surrounds and protects the bony prominences of the buttocks. This problem is compounded by poor circulation and an inability to reposition during seating. Pressure ulcers cause severe discomfort and may pose a serious health risk to the wheelchair user.
Attempts have been made to provide wheelchair seating that reduces the likelihood of pressure ulcers. Seat cushions have been developed in an effort to eliminate pressure hotspots (i.e., a center of high pressure within a larger area of relatively low pressure) by pressure redistribution at the bony prominences in contact with a cushion.
One family of cushions uses a fluid membrane sack in conjunction with a foam cushion base as a means of supporting the user. The fluid membrane sack, which contains a gel or liquid-like substance, is intended to support the buttocks under hydrostatic (or otherwise generally evenly distributed) pressure. The fluid is typically contained within one or more thin pliable membrane sacks. The fluid is permitted to flow around the contour of the buttocks, with the bony prominences supported by the fluid, so that the entire region supported by the fluid experiences substantially the same level of pressure.
Another family of cushions uses an air-cell bladder in concert with a foam cushion base as a means of supporting the user. The air-cell bladder is intended to support the buttocks by evenly distributing air pressure in the air-cells in much the same way as with the fluid membrane sack.
One limitation to the efficacy of both of these families of cushions is the transition zone that exists at the region in which the bladder/sack and the foam base converge. When two different materials are used in a cushion, and more particularly on the seating surface of the cushion, the points wherein these two materials come in contact with each other or are attached to each other create a transition area. Generally, the physical properties of these two materials are significantly different in the transition areas and the performance and comfort of the cushion is reduced. When the user sits on a cushion, the cushion base and cushion insert both compress, but, due to the difference in physical properties that exist between the cushion base and cushion insert, they compress differently and produce uncomfortable differences in pressure on the user's body.
Current cushions focus on either the type of pressure relieving mediums (gel, fluids, air) or the types and shapes of the cushion foam base.